Classical Ciphers

Classical ciphers are the historical encryption techniques that laid the groundwork for modern cryptography. While they are no longer secure by today's standards, understanding them is essential for grasping the principles that underpin modern algorithms.

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Substitution Ciphers

Substitution ciphers replace each letter in the plaintext with another letter or symbol.

Caesar Cipher

The Caesar cipher is one of the oldest known encryption techniques, used by Julius Caesar around 50 BC:

• Each letter is shifted by a fixed number of positions in the alphabet
• With a shift of 3: A becomes D, B becomes E, C becomes F, and so on

Plaintext:  HELLO WORLD
Shift:      3
Ciphertext: KHOOR ZRUOG

Why it is insecure:

• Only 25 possible shifts — all can be tried in seconds (brute force)
• The key space is far too small

ROT13

ROT13 is a special case of the Caesar cipher with a shift of 13:

A ↔ N    B ↔ O    C ↔ P    ...    M ↔ Z

Applying ROT13 twice returns the original text. It is used for trivial obfuscation (e.g., hiding spoilers) but provides zero security.

Monoalphabetic Substitution Cipher